The federal agency that will play a pivotal role in guiding the sentence of former Virginia governor Robert F. McDonnell has recommended that the onetime Republican rising star spend at least 10 years and a month in prison, according to several people familiar with the matter.

The guidelines recommended by the U.S. probation office are preliminary, and even if finalized, U.S. District Judge James R. Spencer is not required to follow them. But experts said that Spencer typically heeds the probation office’s advice, and judges in his district have imposed sentences within the recommendations more than 70 percent of the time in recent years.

“It’s of critical importance,” said Scott Fredericksen, a white-collar criminal defense lawyer. “The fact is, the vast majority of times, courts follow those recommendations closely.”

The matter is far from settled. The probation office recommended a punishment from 10 years and a month to 12 years and 7 months. Calculating an appropriate range of sentences in the federal system is a complicated, mathematical process that takes into account a variety of factors, including the type of crime, the defendant’s role and the amount of loss. The judge has yet to see the arguments from each side.

McDonnell and his wife, Maureen, were convicted in September of lending the prestige of his office to Richmond businessman Jonnie R. Williams Sr. in exchange for $177,000 in loans, vacations and luxury items.

List of gifts given to the McDonnell family from Jonnie Williams.

McDonnell is scheduled to be sentenced Jan. 6. His wife’s sentencing is scheduled for Feb. 20, and her guideline range is expected to be lower than her husband’s. The probation office has not yet filed a report concerning her.

It is unclear how the probation office determined that the former governor’s crimes necessitate a minimum decade-long sentence. The initial report on the matter is sealed, and people familiar with its contents revealed only the recommended range to The Washington Post.

The range is particularly notable because last December, prosecutors offered to let McDonnell plead guilty to just one count of lying to a bank as part of an agreement that would have meant he could be sentenced to three years in prison at the most and probation at the least. Importantly, though, McDonnell would have been required to sign a statement acknowledging that he helped Star Scientific, Williams’s dietary-supplement company, at the same time the businessman was giving him loot, fully shouldering blame for a relationship he has insisted was not criminal and was driven largely by his wife.

A federal probation official in Richmond, U.S. Attorney Dana Boente and a McDonnell defense attorney all declined to comment for this article.

White-collar criminal defense lawyer Matthew Kaiser said McDonnell’s range probably was increased because he was a high-ranking public official, because he took more than one payment from Williams and because the total value of the gifts he received was so high. Kaiser said the probation officer also probably faulted McDonnell because his testimony was contrary to the jury’s verdict.

Prosecutors and defense attorneys will still have an opportunity to argue to the probation officer about whether the range was correctly calculated — although Kaiser said the probation office often “sticks to its guns.” After that, both sides can try to persuade Spencer to modify the recommended range.

Even then, Spencer is not bound by the guideline. Defense attorneys have already begun working vigorously in their bid to sway him toward leniency. This week, they won a legal skirmish with prosecutors so they can file additional pages in their sentencing memorandum — a key document outlining the sentence they believe McDonnell should receive and why.

It is unclear whether their efforts to move Spencer away from the probation office’s recommended range will be fruitful.

In the Eastern District of Virginia, where McDonnell is being sentenced, judges imposed sentences within the guideline range more than 70 percent of the time last fiscal year, according to data from the U.S. Sentencing Commission. In about 21 percent of cases, they imposed sentences below the guideline range without a request from prosecutors to do so.

Nationally, judges imposed sentences within the guideline range about 51 percent of the time last fiscal year and deviated downward without a request from prosecutors to do so in about 19 percent of cases.

In the McDonnell case, prosecutors are not expected to ask for a sentence below the guideline range.

White-collar criminal defense lawyer Jacob Frenkel said Spencer might vary from the guidelines in McDonnell’s case — taking into account, particularly, the good he has done for Virginians — but is unlikely to apply “the extent of the discretion the defendants would hope for.”

Brian Whisler, a defense lawyer who used to work as a federal prosecutor in Richmond, said that Spencer is known to be “largely deferential to the probation office and its sentencing calculations.” Whisler — whose firm, Baker & McKenzie, represented state employees in the McDonnell case — said the judge will likely draw on other cases in the district to inform his conclusion.

The outcome of those might not be to McDonnell’s liking. In 2011, another federal judge in Richmond sentenced former Virginia delegate Phillip A. Hamilton to  91/2 years in prison in a bribery and extortion case. In 2009, a federal judge in Alexandria sentenced former congressman William J. Jefferson to 13 years in prison for accepting hundreds of thousands of dollars in bribes — though, notably, that fell well short of the recommended range of 27 to 33 years.

Rosalind S. Helderman contributed to this report.